InuYasha Episode 57: Fateful Night in Togenkyo, Part I
Episode 57 of 167 of InuYasha, aired January 21, 2002, 24 minutes. Available with English subtitles and dub where released.
Episode summary
After Sango leaves to repair her damaged Hiraikotsu, Inuyasha and the others come across a demon tree and a sage known as Tokajin, who uses humans to feed the tree so it can grow a fruit of immortality. Not only do the four manage to be caught by Tokajin in some way, but it's also the night of the new moon, when Inuyasha becomes human.